The transforming growth factor 2 (TGF2) signaling controls many cellular processes including cell proliferation and apoptosis and is critical for maintaining tissue homeostasis. Aberrant levels of TGF2 signaling lead to defective tissue regeneration and various human diseases such as cancer. The long-term goal of this project is to understand how proper levels of TGF2 signaling activity are maintained and regulated in normal tissues and primary cells using a system biology approach. The goal of this application is to develop a comprehensive mathematical model that can be used to predict Smad signaling dynamics and behavior and to determine the function of SnoN-mediated negative feedback regulation of Smad signaling. We will employ primary hepatocytes from wild type and SnoN-null mice to perform quantitative measurement and establish a mathematical model, and further confirm the model in liver tissue sections. We choose to work with liver because TGF2 is known to be a potent inhibitor of hepatocyte proliferation and is also the critical negative regulator of liver regeneration. In primary hepatocytes, TGF2 signals through Smad2 and Smad4 proteins. Upon phosphorylation by the active TGFss receptor kinases, Smad2 homo-oligomerizes and heterooligomerizes with Smad4, leading to its accumulation in the nucleus. The heteromeric Smad2/Smad4 complex then interacts with other transcription factors to regulate expression of TGF2-responsive genes. The activities of the Smad proteins are regulated by positive and negative cellular co-factors. Through a system biology study, we have recently determined that SnoN is the most important negative regulator of Smad signaling in primary hepatocytes. SnoN binds to Smad2 and Smad4 and represses their ability to activate TGF2 target genes. Interestingly, SnoN itself is induced by TGF2, suggesting that it modulates TGF2 signaling in a negative feedback manner. Our preliminary study suggests that SnoN neither affects the duration nor the initial timing of Smad signaling, but rather decreases the level of target gene expression at high TGF2 concentrations. We hypothesize that the SnoN-mediated negative feedback functions to suppress cell-to-cell variability in Smad target gene expression and linearize dose-response behavior of TGF2 signaling, thereby maintaining the robustness of this signaling pathway. In this proposal we will combine mathematical modeling with quantitative measurements to test this hypothesis at both the cellular level, using primary hepatocytes, and at the organ level, using liver tissues from wild type and snoN mutant mice. Three specific aims have been proposed: 1) Analysis of the impact of SnoN on TGF2 signaling in primary hepatocytes. 2) Analysis of the effects of SnoN on Smad-dependent gene expression. 3) Modeling SnoN regulation of Smad signaling during liver damage and regeneration. This study will significantly advance our knowledge regarding the mechanism and regulation of this important signaling pathway and how de-regulation of this signaling activity results in carcinogenesis.